The present invention relates to a process for emulsion-polymerizing an acrylate or a methacrylate (hereinafter referred to as "(meth)acrylate"), more particularly relates to a process of polymerization capable of preventing scale deposition by carrying out an emulsion-polymerization of (meth)acrylate monomers in an aqueous medium within a polymerization vessel on an inner surface of which a film of an aromatic quinone-amine compound which is insoluble in the monomers is formed.
An emulsion polymerization using a (meth)acrylate monomer is popularly applied to a process for preparing an acrylonitrile-butadiene-styrene copolymer (ABS) or a methyl methacrylate-butadiene-styrene copolymer(MBS). In such a process, the polymerization is carried out in a manner of graft polymerization to a butadiene rubber latex. As a result of the polymerization, since the formed polymer scale is adhesive, the scale is hardly removed from a surface of a polymerization vessel.
The deposition of the polymer scale is assumed to occur in the following manner. The (meth)acrylate monomers are dissolved in water, and then polymerized in water, which makes the latex unstable. In addition, since the (meth)acrylate monomers are hydrolyzed to acidify the polymerization system weakly, agglomeration of the latex is promoted. For preventing the latex from the agglomeration, a proper emulsifier is selected. It is very difficult, however, to carry out the polymerization in a stable state of the latex, because the quality of the produced polymer becomes degraded by the increase of the amount of the emulsifier. Therefore the partial agglomeration of the latex cannot be avoided.
The agglomerated latex is dissolved in the (meth)acrylate monomers to increase in stickness. The sticky agglomerated latex is adhered on surfaces of various parts in a polymerization vessel, and then the (meth)acrylate monomers contained in the latex are polymerized there to form a tough polymer scale deposit.
This causes many defects such as decrease in heat transfer efficiency through the inner wall of the vessel, decrease in yield, deterioration of products due to the admixing of the pealed scales, and reduction in oparating rate of the vessel due to labors and time required for removing the scales from the surface.
There hitherto have been proposed various processes for preventing polymer scale deposition, for instance, a process in which a polymerization vessel is prevented from scale deposition by previously applying a quinone-amine compound to the surfaces of the vessel to form a film of the compound, and then carrying out the polymerication. The process is applicable to a dispersion polymerization of a halogenated vinyl monomer such as vinyl chloride or a monomer mixture of the hologenated vinyl monomer containing a small amount of a copolymerizable monomer therewith.
In a polymerization system in which the amount of (meth)acrylate monomers become predominant in a course of the polymerization, however, it has been thought that the quinone-amine compound film is not effective for the prevention of the polymer scale deposition, because a solubility power of the (meth)acrylate monomers is extremely larger than that of the halogenated vinyl monomer. Namely, the quinone-amine compound film used for the polymerization of the halogenated vinyl monomer system is partially or wholly dissolved in the (meth)acrylate monomers. As a result, there arise defects that the scale deposition preventing effect is remarkably decreased and that the product is contaminated by the dissolved quinone-amine compound and is colored. The solubility power of the (meth)acrylate monomers is larger than that of an aromatic monomer such as styrene or .alpha.-methylstyrene.
In general, the solubility power of the (meth)acrylate monomers is lowered in the presence of a halogenated vinyl monomer which is a poor solvent for the quinone-amine compound. However, when the (meth)acrylate monomers exist in a monomer mixture at not less than 60% (% by weight, hereinafter the same), the above quinone-amine compound is almost dissolved in the monomer mixture, and thus the scale deposition preventing effect cannot be obtained. Accordingly the conventional quinone-amine compound used for the dispersion polymerization system of the halogenated vinyl monomer cannot be employed in the emulsion polymerization system containing the (meth)acrylate monomers in an amount of not less than 60%.
From the above-mentioned reason and the fact that an industrial process concerning the polymerization of the (meth)acrylate monomers are usually carried out in the above-mentioned condition where the amount of the (meth)acrylate monomers becomes not less than 60%, there is no useful scale deposition preventing agent for the emulsion polymerization of the (meth)acrylate monomer system.
As a result of the present inventors' study for improving a scale deposition preventing agent useful in the (meth)acrylate monomer system, it has been found that a quinone-amine compound prepared under the specific condition has an excellent scale deposition preventing effect in the (meth)acrylate monomer system, and then the present invention has been completed.